“Everyone knows how intense Sean is, and there is no exception on movie sets. Charlize felt he was overly critical of her at times. Separating work and personal became hard, and the lines were definitely blurred,” the source noted.

The split came as a shock to many as both Charlize, 39, and 54-year-old Sean had spoken about each other lovingly in recent months.

Sean Penn (right) and partner Charlize Theron attend the World Premiere of The Gunman (PA Wire)

Sean, who’s been married twice before, told British Esquire in March how much he liked the tradition of a legal union.

"You say I've been married twice before, but I’ve been married under circumstances where I was less informed than I am today,” he said. "So I wouldn’t even consider it a third marriage, I’d consider it a first marriage on its own terms if I got married again. I mean, I like the tradition. A friend of mine wrote a line, 'Without tradition, new things die.' And I don’t want new things to die."